On 11/12/20 6:06 PM, Shaokun Zhang wrote:
>>> On Huawei Kunpeng 920 server, there are 4 NUMA node(0 - 3) in the 2-cpu
>>> system(0 - 1). The topology of this server is followed:
>>
>> This is with a feature enabled that Intel calls sub-NUMA-clustering
>> (SNC), right? Explaining *that* feature would also be great context for
>
> Correct,
>
>> why this gets triggered on your system and not normally on others and
>> why nobody noticed this until now.
>
> This is on intel 6248 platform:
I have no idea what a "6248 platform" is.
>>> +static void calc_node_distance(int *node_dist, int node)
>>> +{
>>> + int i;
>>> +
>>> + for (i = 0; i < nr_node_ids; i++)
>>> + node_dist[i] = node_distance(node, i);
>>> +}
>>
>> This appears to be the only place node_dist[] is written. That means it
>> always contains a one-dimensional slice of the two-dimensional data
>> represented by node_distance().
>>
>> Why is a copy of this data needed?
>
> It is used to store the distance with the @node for later, apologies that I
> can't follow your question correctly.
Right, the data that you store is useful. *But*, it's also a verbatim
copy of the data from node_distance(). Why not just use node_distance()
directly in your code rather than creating a partial copy of it in the
local node_dist[] array?
>>> unsigned int cpumask_local_spread(unsigned int i, int node)
>>> {
>>> - int cpu, hk_flags;
>>> + static DEFINE_SPINLOCK(spread_lock);
>>> + static int node_dist[MAX_NUMNODES];
>>> + static bool used[MAX_NUMNODES];
>>
>> Not to be *too* picky, but there is a reason we declare nodemask_t as a
>> bitmap and not an array of bools. Isn't this just wasteful?
>>
>>> + unsigned long flags;
>>> + int cpu, hk_flags, j, id;
>>> const struct cpumask *mask;
>>>
>>> hk_flags = HK_FLAG_DOMAIN | HK_FLAG_MANAGED_IRQ;
>>> @@ -220,20 +256,28 @@ unsigned int cpumask_local_spread(unsigned int i, int
>>> node)
>>> return cpu;
>>> }
>>> } else {
>>> - /* NUMA first. */
>>> - for_each_cpu_and(cpu, cpumask_of_node(node), mask) {
>>> - if (i-- == 0)
>>> - return cpu;
>>> - }
>>> + spin_lock_irqsave(&spread_lock, flags);
>>> + memset(used, 0, nr_node_ids * sizeof(bool));
>>> + calc_node_distance(node_dist, node);
>>> + /* Local node first then the nearest node is used */
>>
>> Is this comment really correct? This makes it sound like there is only
>
> I think it is correct, that's what we want to choose the nearest node.
>
>> fallback to a single node. Doesn't the _code_ fall back basically
>> without limit?
>
> If I follow your question correctly, without this patch, if the local
> node is used up, one random node will be choosed, right? Now we firstly
> choose the nearest node by the distance, if all nodes has been choosen,
> it will return the initial solution.
The comment makes it sound like the code does:
1. Do the local node
2. Do the next nearest node
3. Stop
In reality, I *think* it's more of a loop where it search
ever-increasing distances away from the local node.
I just think the comment needs to be made more precise.
>>> + for (j = 0; j < nr_node_ids; j++) {
>>> + id = find_nearest_node(node_dist, used);
>>> + if (id < 0)
>>> + break;
>>>
>>> - for_each_cpu(cpu, mask) {
>>> - /* Skip NUMA nodes, done above. */
>>> - if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
>>> - continue;
>>> + for_each_cpu_and(cpu, cpumask_of_node(id), mask)
>>> + if (i-- == 0) {
>>> + spin_unlock_irqrestore(&spread_lock,
>>> + flags);
>>> + return cpu;
>>> + }
>>> + used[id] = 1;
>>> + }
>>> + spin_unlock_irqrestore(&spread_lock, flags);
>>
>> The existing code was pretty sparsely commented. This looks to me to
>> make it more complicated and *less* commented. Not the best combo.
>
> Apologies for the bad comments, hopefully I describe it clearly by the above
> explantion.
Do you want to take another pass at submitting this patch?
